refactor(month): extract per-week layout and share the agenda's rows

Groundwork for the month view style setting (#38, #53); no behaviour change.

- Split layoutCalendarWeek out of layoutMonthWeeks so the continuous style,
  which streams weeks and has no enclosing YearMonth to slice by, lays rows
  out identically. layoutMonthWeeks (also used by the month widget) keeps its
  signature and becomes a loop over it.
- Add MonthUiState.Success.instancesByDay: the grid's events keyed by date and
  uncapped, so the split style's day pane can list a date without a second
  provider query — the month grid range already covers it.
- Move AgendaDayHeader / AgendaEmptyDayRow / AgendaEventRow and their label
  helpers into AgendaRows.kt as internal, so the split pane reuses the agenda's
  row vocabulary instead of growing a parallel one. AgendaEmptyDayRow takes its
  text as a parameter now that it serves more than "nothing left today".
- Add the month package's first JVM tests, covering week counts, span
  continuation across row boundaries, lane stacking and instancesByDay.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/ui/agenda/AgendaRows.kt

@@ -0,0 +1,206 @@
+package de.jeanlucmakiola.calendula.ui.agenda
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.isSystemInDarkTheme
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.Coffee
+import androidx.compose.material3.Card
+import androidx.compose.material3.Icon
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.alpha
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.dp
+import de.jeanlucmakiola.calendula.R
+import de.jeanlucmakiola.calendula.domain.EventInstance
+import de.jeanlucmakiola.calendula.ui.common.EventDimAlpha
+import de.jeanlucmakiola.calendula.ui.common.LocalSoftenColors
+import de.jeanlucmakiola.calendula.ui.common.LocalUse24HourFormat
+import de.jeanlucmakiola.calendula.ui.common.eventFill
+import de.jeanlucmakiola.calendula.ui.common.formatTimeOfDay
+import de.jeanlucmakiola.floret.components.GroupedRow
+import de.jeanlucmakiola.floret.components.Position
+import de.jeanlucmakiola.floret.locale.currentLocale
+import de.jeanlucmakiola.floret.locale.localizedDateFormatter
+import kotlinx.datetime.DateTimeUnit
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.plus
+import kotlinx.datetime.toLocalDateTime
+import kotlin.time.Instant
+import java.util.Locale
+
+// The agenda's row vocabulary, split out of AgendaScreen so the month view's
+// split style can list a day with exactly the same visual language instead of
+// growing a parallel set of event rows.
+
+@Composable
+internal fun AgendaDayHeader(
+    date: LocalDate,
+    today: LocalDate,
+    onOpenDay: (LocalDate) -> Unit,
+) {
+    Surface(
+        color = MaterialTheme.colorScheme.surface,
+        modifier = Modifier
+            .fillMaxWidth()
+            .clickable { onOpenDay(date) },
+    ) {
+        Text(
+            text = agendaDayLabel(date, today),
+            style = MaterialTheme.typography.titleSmall,
+            color = if (date == today) {
+                MaterialTheme.colorScheme.primary
+            } else {
+                MaterialTheme.colorScheme.onSurfaceVariant
+            },
+            modifier = Modifier.padding(start = 28.dp, end = 28.dp, top = 16.dp, bottom = 8.dp),
+        )
+    }
+}
+
+/**
+ * A card standing in for a day with no events — the same coffee-cup motif as the
+ * agenda's full-screen empty state, boxed into a card so the day keeps a visible slot
+ * rather than a bare header. Used for an anchored, event-less today (#35) and for
+ * an empty selected day in the month view's split style.
+ */
+@Composable
+internal fun AgendaEmptyDayRow(text: String, onClick: () -> Unit) {
+    Card(
+        // Match a single event row's resting corner radius (floret groupedShape).
+        shape = RoundedCornerShape(22.dp),
+        modifier = Modifier
+            .fillMaxWidth()
+            .padding(horizontal = 16.dp, vertical = 4.dp)
+            .clickable(onClick = onClick),
+    ) {
+        Column(
+            modifier = Modifier
+                .fillMaxWidth()
+                .padding(vertical = 18.dp, horizontal = 16.dp),
+            horizontalAlignment = Alignment.CenterHorizontally,
+        ) {
+            Icon(
+                imageVector = Icons.Filled.Coffee,
+                contentDescription = null,
+                tint = MaterialTheme.colorScheme.onSurfaceVariant,
+                modifier = Modifier.size(36.dp),
+            )
+            Spacer(Modifier.height(8.dp))
+            Text(
+                text = text,
+                style = MaterialTheme.typography.bodyLarge,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                textAlign = TextAlign.Center,
+            )
+        }
+    }
+}
+
+@Composable
+internal fun AgendaEventRow(
+    event: EventInstance,
+    day: LocalDate,
+    zone: TimeZone,
+    position: Position,
+    dimmed: Boolean,
+    modifier: Modifier = Modifier,
+    onClick: () -> Unit,
+) {
+    val dark = isSystemInDarkTheme()
+    val soften = LocalSoftenColors.current
+    val title = event.title.ifBlank { stringResource(R.string.event_untitled) }
+    GroupedRow(
+        modifier = if (dimmed) modifier.alpha(EventDimAlpha) else modifier,
+        title = title,
+        summary = agendaTimeSummary(event, day, zone),
+        position = position,
+        minHeight = 64.dp,
+        leading = {
+            Box(
+                modifier = Modifier
+                    .size(width = 6.dp, height = 36.dp)
+                    .clip(RoundedCornerShape(3.dp))
+                    .background(eventFill(event.color, dark, soften)),
+            )
+        },
+        onClick = onClick,
+    )
+}
+
+/** "Today · Wed, 17. Jun 2026" — relative word for today/tomorrow, else the date. */
+@Composable
+internal fun agendaDayLabel(date: LocalDate, today: LocalDate): String {
+    val relative = when (date) {
+        today -> stringResource(R.string.agenda_header_today)
+        today.plus(1, DateTimeUnit.DAY) -> stringResource(R.string.agenda_header_tomorrow)
+        else -> null
+    }
+    val formatted = formatAgendaDate(date)
+    return if (relative != null) "$relative · $formatted" else formatted
+}
+
+/**
+ * Time line under the title: "09:00 – 10:00 · Location", "All day", etc.
+ *
+ * A multi-day event shows only the part relevant to [day], spelled out so each
+ * day reads on its own: its first day names the start ("Starts 14:00"), its last
+ * day the end ("Ends 10:00"), and any whole day in between reads as "All day".
+ * An all-day multi-day event is simply "All day" on every day it covers.
+ */
+@Composable
+internal fun agendaTimeSummary(event: EventInstance, day: LocalDate, zone: TimeZone): String {
+    val is24Hour = LocalUse24HourFormat.current
+    val locale = currentLocale()
+
+    val time = when (val label = agendaTimeLabel(event, day, zone)) {
+        AgendaTimeLabel.AllDay -> stringResource(R.string.event_detail_all_day)
+        is AgendaTimeLabel.Starts -> stringResource(
+            R.string.agenda_span_starts,
+            formatTime(label.start, zone, is24Hour, locale),
+        )
+        is AgendaTimeLabel.Ends -> stringResource(
+            R.string.agenda_span_ends,
+            formatTime(label.end, zone, is24Hour, locale),
+        )
+        is AgendaTimeLabel.Range -> "${formatTime(label.start, zone, is24Hour, locale)} – " +
+            formatTime(label.end, zone, is24Hour, locale)
+    }
+
+    val location = event.location?.takeIf { it.isNotBlank() }
+    return if (location != null) "$time · $location" else time
+}
+
+private fun formatTime(
+    instant: Instant,
+    zone: TimeZone,
+    is24Hour: Boolean,
+    locale: Locale,
+): String {
+    val t = instant.toLocalDateTime(zone).time
+    return formatTimeOfDay(t.hour, t.minute, is24Hour, locale)
+}
+
+private fun formatAgendaDate(date: LocalDate): String {
+    val locale = Locale.getDefault()
+    val java = java.time.LocalDate.of(date.year, date.month.ordinal + 1, date.day)
+    // Weekday + date in the locale's own field order (e.g. "Wed, Jun 17, 2026"
+    // vs "Mi., 17. Juni 2026") rather than a hardcoded day-month-year layout.
+    return localizedDateFormatter(locale, "EEEdMMMy").format(java)
+}

app/src/main/java/de/jeanlucmakiola/calendula/ui/agenda/AgendaScreen.kt

@@ -1,9 +1,6 @@
 package de.jeanlucmakiola.calendula.ui.agenda
 
 import androidx.compose.foundation.ExperimentalFoundationApi
-import androidx.compose.foundation.background
-import androidx.compose.foundation.clickable
-import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -17,13 +14,11 @@ import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.itemsIndexed
-import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.Coffee
 import androidx.compose.material.icons.filled.Menu
 import androidx.compose.material.icons.filled.Search
 import androidx.compose.material3.ButtonDefaults
-import androidx.compose.material3.Card
 import androidx.compose.material3.DrawerValue
 import androidx.compose.material3.FilledTonalButton
 import androidx.compose.material3.ExperimentalMaterial3Api
@@ -32,7 +27,6 @@ import androidx.compose.material3.IconButton
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.ModalNavigationDrawer
 import androidx.compose.material3.Scaffold
-import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.material3.TopAppBar
 import androidx.compose.material3.TopAppBarDefaults
@@ -46,8 +40,6 @@ import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
-import androidx.compose.ui.draw.alpha
-import androidx.compose.ui.draw.clip
 import androidx.compose.ui.input.nestedscroll.nestedScroll
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.text.style.TextAlign
@@ -59,8 +51,6 @@ import de.jeanlucmakiola.calendula.data.prefs.PastEventDisplay
 import de.jeanlucmakiola.calendula.domain.EventInstance
 import de.jeanlucmakiola.calendula.domain.hasEnded
 import de.jeanlucmakiola.calendula.ui.common.AgendaRangePicker
-import de.jeanlucmakiola.calendula.ui.common.LocalSoftenColors
-import de.jeanlucmakiola.calendula.ui.common.eventFill
 import de.jeanlucmakiola.calendula.ui.common.agendaRangeLabel
 import de.jeanlucmakiola.floret.identity.animateItemMotion
 import de.jeanlucmakiola.calendula.ui.common.CalendarDrawer
@@ -69,25 +59,15 @@ import de.jeanlucmakiola.calendula.ui.common.TodayAction
 import de.jeanlucmakiola.calendula.ui.common.CalendarFailure
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
 import de.jeanlucmakiola.calendula.ui.common.IMPLEMENTED_VIEWS
-import de.jeanlucmakiola.floret.locale.localizedDateFormatter
-import de.jeanlucmakiola.calendula.ui.common.EventDimAlpha
-import de.jeanlucmakiola.floret.components.GroupedRow
-import de.jeanlucmakiola.floret.components.Position
 import de.jeanlucmakiola.calendula.ui.common.ViewSwitcherPill
 import de.jeanlucmakiola.calendula.ui.common.next
 import de.jeanlucmakiola.floret.components.positionOf
 import de.jeanlucmakiola.calendula.ui.common.rememberCurrentMinute
 import de.jeanlucmakiola.floret.locale.currentLocale
-import de.jeanlucmakiola.calendula.ui.common.LocalUse24HourFormat
-import de.jeanlucmakiola.calendula.ui.common.formatTimeOfDay
 import kotlinx.coroutines.launch
-import kotlinx.datetime.DateTimeUnit
 import kotlinx.datetime.LocalDate
 import kotlinx.datetime.TimeZone
-import kotlinx.datetime.plus
-import kotlinx.datetime.toLocalDateTime
 import kotlin.time.Instant
-import java.util.Locale
 
 // No file-level zone constant here on purpose: it would be fixed for the process
 // lifetime and drift from the zone AgendaViewModel groups in after a device
@@ -370,7 +350,10 @@ private fun AgendaList(
             if (day.events.isEmpty()) {
                 // An anchored, event-less today (#35) — "nothing left today".
                 item(key = "placeholder-${day.date}") {
-                    AgendaEmptyDayRow(onClick = { onOpenDay(day.date) })
+                    AgendaEmptyDayRow(
+                        text = stringResource(R.string.agenda_no_more_today),
+                        onClick = { onOpenDay(day.date) },
+                    )
                 }
             } else {
                 itemsIndexed(
@@ -396,100 +379,6 @@ private fun AgendaList(
     }
 }
 
-@Composable
-private fun AgendaDayHeader(
-    date: LocalDate,
-    today: LocalDate,
-    onOpenDay: (LocalDate) -> Unit,
-) {
-    Surface(
-        color = MaterialTheme.colorScheme.surface,
-        modifier = Modifier
-            .fillMaxWidth()
-            .clickable { onOpenDay(date) },
-    ) {
-        Text(
-            text = agendaDayLabel(date, today),
-            style = MaterialTheme.typography.titleSmall,
-            color = if (date == today) {
-                MaterialTheme.colorScheme.primary
-            } else {
-                MaterialTheme.colorScheme.onSurfaceVariant
-            },
-            modifier = Modifier.padding(start = 28.dp, end = 28.dp, top = 16.dp, bottom = 8.dp),
-        )
-    }
-}
-
-/**
- * A card under an anchored, event-less today (#35) — the same coffee-cup motif
- * as the full-screen [AgendaEmpty] state, boxed into a card so today keeps a
- * visible slot when nothing is left rather than a bare header.
- */
-@Composable
-private fun AgendaEmptyDayRow(onClick: () -> Unit) {
-    Card(
-        // Match a single event row's resting corner radius (floret groupedShape).
-        shape = RoundedCornerShape(22.dp),
-        modifier = Modifier
-            .fillMaxWidth()
-            .padding(horizontal = 16.dp, vertical = 4.dp)
-            .clickable(onClick = onClick),
-    ) {
-        Column(
-            modifier = Modifier
-                .fillMaxWidth()
-                .padding(vertical = 18.dp, horizontal = 16.dp),
-            horizontalAlignment = Alignment.CenterHorizontally,
-        ) {
-            Icon(
-                imageVector = Icons.Filled.Coffee,
-                contentDescription = null,
-                tint = MaterialTheme.colorScheme.onSurfaceVariant,
-                modifier = Modifier.size(36.dp),
-            )
-            Spacer(Modifier.height(8.dp))
-            Text(
-                text = stringResource(R.string.agenda_no_more_today),
-                style = MaterialTheme.typography.bodyLarge,
-                color = MaterialTheme.colorScheme.onSurfaceVariant,
-                textAlign = TextAlign.Center,
-            )
-        }
-    }
-}
-
-@Composable
-private fun AgendaEventRow(
-    event: EventInstance,
-    day: LocalDate,
-    zone: TimeZone,
-    position: Position,
-    dimmed: Boolean,
-    modifier: Modifier = Modifier,
-    onClick: () -> Unit,
-) {
-    val dark = isSystemInDarkTheme()
-    val soften = LocalSoftenColors.current
-    val title = event.title.ifBlank { stringResource(R.string.event_untitled) }
-    GroupedRow(
-        modifier = if (dimmed) modifier.alpha(EventDimAlpha) else modifier,
-        title = title,
-        summary = agendaTimeSummary(event, day, zone),
-        position = position,
-        minHeight = 64.dp,
-        leading = {
-            Box(
-                modifier = Modifier
-                    .size(width = 6.dp, height = 36.dp)
-                    .clip(RoundedCornerShape(3.dp))
-                    .background(eventFill(event.color, dark, soften)),
-            )
-        },
-        onClick = onClick,
-    )
-}
-
 @Composable
 private fun AgendaEmpty(modifier: Modifier = Modifier) {
     Column(
@@ -559,64 +448,3 @@ private fun AgendaTopBar(
         scrollBehavior = scrollBehavior,
     )
 }
-
-/** "Today · Wed, 17. Jun 2026" — relative word for today/tomorrow, else the date. */
-@Composable
-private fun agendaDayLabel(date: LocalDate, today: LocalDate): String {
-    val relative = when (date) {
-        today -> stringResource(R.string.agenda_header_today)
-        today.plus(1, DateTimeUnit.DAY) -> stringResource(R.string.agenda_header_tomorrow)
-        else -> null
-    }
-    val formatted = formatAgendaDate(date)
-    return if (relative != null) "$relative · $formatted" else formatted
-}
-
-/**
- * Time line under the title: "09:00 – 10:00 · Location", "All day", etc.
- *
- * A multi-day event shows only the part relevant to [day], spelled out so each
- * day reads on its own: its first day names the start ("Starts 14:00"), its last
- * day the end ("Ends 10:00"), and any whole day in between reads as "All day".
- * An all-day multi-day event is simply "All day" on every day it covers.
- */
-@Composable
-private fun agendaTimeSummary(event: EventInstance, day: LocalDate, zone: TimeZone): String {
-    val is24Hour = LocalUse24HourFormat.current
-    val locale = currentLocale()
-
-    val time = when (val label = agendaTimeLabel(event, day, zone)) {
-        AgendaTimeLabel.AllDay -> stringResource(R.string.event_detail_all_day)
-        is AgendaTimeLabel.Starts -> stringResource(
-            R.string.agenda_span_starts,
-            formatTime(label.start, zone, is24Hour, locale),
-        )
-        is AgendaTimeLabel.Ends -> stringResource(
-            R.string.agenda_span_ends,
-            formatTime(label.end, zone, is24Hour, locale),
-        )
-        is AgendaTimeLabel.Range -> "${formatTime(label.start, zone, is24Hour, locale)} – " +
-            formatTime(label.end, zone, is24Hour, locale)
-    }
-
-    val location = event.location?.takeIf { it.isNotBlank() }
-    return if (location != null) "$time · $location" else time
-}
-
-private fun formatTime(
-    instant: Instant,
-    zone: TimeZone,
-    is24Hour: Boolean,
-    locale: Locale,
-): String {
-    val t = instant.toLocalDateTime(zone).time
-    return formatTimeOfDay(t.hour, t.minute, is24Hour, locale)
-}
-
-private fun formatAgendaDate(date: LocalDate): String {
-    val locale = Locale.getDefault()
-    val java = java.time.LocalDate.of(date.year, date.month.ordinal + 1, date.day)
-    // Weekday + date in the locale's own field order (e.g. "Wed, Jun 17, 2026"
-    // vs "Mi., 17. Juni 2026") rather than a hardcoded day-month-year layout.
-    return localizedDateFormatter(locale, "EEEdMMMy").format(java)
-}

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthUiState.kt

@@ -40,9 +40,17 @@ data class MonthWeek(
 sealed interface MonthUiState {
     data object Loading : MonthUiState
     data class Failure(val reason: FailureReason) : MonthUiState
+
+    /**
+     * [weeks] is what the grid draws; [instancesByDay] is the same events keyed by
+     * date and *uncapped*, so the split style's day pane can list everything on a
+     * date without a second provider query — the month grid range already covers
+     * it. All-day events sort first, then by start.
+     */
     data class Success(
         val month: YearMonth,
         val today: LocalDate,
         val weeks: List<MonthWeek>,
+        val instancesByDay: Map<LocalDate, List<EventInstance>> = emptyMap(),
     ) : MonthUiState
 }

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthViewModel.kt

@@ -117,10 +117,12 @@ class MonthViewModel @Inject constructor(
         if (calendars.isEmpty()) {
             return MonthUiState.Failure(FailureReason.NoCalendarsConfigured)
         }
+        val weeks = layoutMonthWeeks(ym, weekStart, instances, zone)
         return MonthUiState.Success(
             month = ym,
             today = todayDate,
-            weeks = layoutMonthWeeks(ym, weekStart, instances, zone),
+            weeks = weeks,
+            instancesByDay = instancesByDay(weeks.flatMap { it.days }, instances, zone),
         )
     }
 }
@@ -149,31 +151,63 @@ internal fun layoutMonthWeeks(
 
     return (0 until weekCount).map { row ->
         val days = (0 until 7).map { gridStart.plus(row * 7 + it, DateTimeUnit.DAY) }
-        val weekEvents = instances.filter { ev -> days.any { ev.coversDay(it, zone) } }
+        layoutCalendarWeek(days, instances, zone)
-        val (bars, singles) = weekEvents.partition { ev ->
-            ev.isAllDay || days.count { ev.coversDay(it, zone) } > 1
-        }
-        val spans = layoutAllDay(bars, days, zone).map { s ->
-            MonthSpan(
-                event = s.event,
-                startCol = s.startCol,
-                endCol = s.endCol,
-                lane = s.lane,
-                continuesLeft = s.event.coversDay(days.first().minus(1, DateTimeUnit.DAY), zone),
-                continuesRight = s.event.coversDay(days.last().plus(1, DateTimeUnit.DAY), zone),
-            )
-        }
-        MonthWeek(
-            days = days,
-            spans = spans,
-            timedByDay = days.associateWith { d ->
-                singles.filter { it.coversDay(d, zone) }.sortedBy { it.start }
-            },
-            countByDay = days.associateWith { d -> weekEvents.count { it.coversDay(d, zone) } },
-        )
     }
 }
 
+/**
+ * Resolve one week row's events for rendering. Split out of [layoutMonthWeeks] so
+ * the continuous style — which streams weeks rather than months and so has no
+ * enclosing [YearMonth] to slice by — lays each row out identically.
+ *
+ * [days] must be the row's seven consecutive dates, in display order.
+ */
+internal fun layoutCalendarWeek(
+    days: List<LocalDate>,
+    instances: List<EventInstance>,
+    zone: TimeZone,
+): MonthWeek {
+    val weekEvents = instances.filter { ev -> days.any { ev.coversDay(it, zone) } }
+    val (bars, singles) = weekEvents.partition { ev ->
+        ev.isAllDay || days.count { ev.coversDay(it, zone) } > 1
+    }
+    val spans = layoutAllDay(bars, days, zone).map { s ->
+        MonthSpan(
+            event = s.event,
+            startCol = s.startCol,
+            endCol = s.endCol,
+            lane = s.lane,
+            continuesLeft = s.event.coversDay(days.first().minus(1, DateTimeUnit.DAY), zone),
+            continuesRight = s.event.coversDay(days.last().plus(1, DateTimeUnit.DAY), zone),
+        )
+    }
+    return MonthWeek(
+        days = days,
+        spans = spans,
+        timedByDay = days.associateWith { d ->
+            singles.filter { it.coversDay(d, zone) }.sortedBy { it.start }
+        },
+        countByDay = days.associateWith { d -> weekEvents.count { it.coversDay(d, zone) } },
+    )
+}
+
+/**
+ * Every event touching each of [days], all-day first then by start time. Unlike
+ * [MonthWeek.timedByDay] this keeps multi-day and all-day events on every date
+ * they cover and applies no display cap, so the split style's day pane can list a
+ * date in full without querying the provider again.
+ */
+internal fun instancesByDay(
+    days: List<LocalDate>,
+    instances: List<EventInstance>,
+    zone: TimeZone,
+): Map<LocalDate, List<EventInstance>> =
+    days.associateWith { day ->
+        instances
+            .filter { it.coversDay(day, zone) }
+            .sortedWith(compareByDescending<EventInstance> { it.isAllDay }.thenBy { it.start })
+    }
+
 /**
  * The on-screen grid spans 6 weeks anchored on [weekStart]. Includes the
  * trailing days of the previous month and the leading days of the next month.

app/src/test/java/de/jeanlucmakiola/calendula/ui/month/MonthLayoutTest.kt

@@ -0,0 +1,220 @@
+package de.jeanlucmakiola.calendula.ui.month
+
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.calendula.domain.EventInstance
+import kotlinx.datetime.DateTimeUnit
+import kotlinx.datetime.DayOfWeek
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.YearMonth
+import kotlinx.datetime.atTime
+import kotlinx.datetime.plus
+import kotlinx.datetime.toInstant
+import kotlin.time.Instant
+import org.junit.jupiter.api.Test
+
+class MonthLayoutTest {
+
+    private val zone = TimeZone.UTC
+
+    // 2026-06-01 is a Monday, so a Monday-anchored June grid starts on the 1st.
+    private val jun = YearMonth(2026, kotlinx.datetime.Month.JUNE)
+    private val jun1 = LocalDate(2026, 6, 1)
+    private val jun8 = LocalDate(2026, 6, 8)
+    private val weekOf8th = (0..6).map { jun8.plus(it, DateTimeUnit.DAY) }
+
+    private fun at(date: LocalDate, h: Int, m: Int = 0): Instant =
+        date.atTime(h, m).toInstant(zone)
+
+    private fun timed(
+        date: LocalDate,
+        startHour: Int,
+        endHour: Int,
+        id: Long = 1L,
+        title: String = "E",
+    ) = EventInstance(
+        instanceId = id,
+        eventId = id,
+        calendarId = 1L,
+        title = title,
+        start = at(date, startHour),
+        end = at(date, endHour),
+        isAllDay = false,
+        color = 0xFF112233.toInt(),
+        location = null,
+    )
+
+    /** All-day events live at UTC midnights with an *exclusive* end. */
+    private fun allDay(
+        from: LocalDate,
+        toInclusive: LocalDate = from,
+        id: Long = 100L,
+        title: String = "A",
+    ) = EventInstance(
+        instanceId = id,
+        eventId = id,
+        calendarId = 1L,
+        title = title,
+        start = from.atTime(0, 0).toInstant(TimeZone.UTC),
+        end = toInclusive.plus(1, DateTimeUnit.DAY).atTime(0, 0).toInstant(TimeZone.UTC),
+        isAllDay = true,
+        color = 0xFF445566.toInt(),
+        location = null,
+    )
+
+    @Test
+    fun `startOfGridWeek snaps back to the configured week start`() {
+        val wed = LocalDate(2026, 6, 10)
+        assertThat(wed.startOfGridWeek(DayOfWeek.MONDAY)).isEqualTo(jun8)
+        assertThat(jun8.startOfGridWeek(DayOfWeek.MONDAY)).isEqualTo(jun8)
+        // A Sunday-anchored week containing the 10th starts on the 7th.
+        assertThat(wed.startOfGridWeek(DayOfWeek.SUNDAY)).isEqualTo(LocalDate(2026, 6, 7))
+    }
+
+    @Test
+    fun `monthGridRange always covers 42 days from the grid start`() {
+        val range = monthGridRange(jun, DayOfWeek.MONDAY, zone)
+        assertThat(range.start).isEqualTo(at(jun1, 0))
+        // 42 days inclusive → the last second of 2026-07-12.
+        assertThat(range.endInclusive)
+            .isEqualTo(LocalDate(2026, 7, 12).atTime(23, 59, 59).toInstant(zone))
+    }
+
+    @Test
+    fun `week count follows the month's shape rather than a fixed six rows`() {
+        // June 2026: starts Monday, 30 days → 5 rows.
+        assertThat(layoutMonthWeeks(jun, DayOfWeek.MONDAY, emptyList(), zone)).hasSize(5)
+        // August 2026: starts Saturday, 31 days → spills to 6 rows.
+        val aug = YearMonth(2026, kotlinx.datetime.Month.AUGUST)
+        assertThat(layoutMonthWeeks(aug, DayOfWeek.MONDAY, emptyList(), zone)).hasSize(6)
+        // February 2021: starts Monday, 28 days → exactly 4 rows.
+        val feb = YearMonth(2021, kotlinx.datetime.Month.FEBRUARY)
+        assertThat(layoutMonthWeeks(feb, DayOfWeek.MONDAY, emptyList(), zone)).hasSize(4)
+    }
+
+    @Test
+    fun `layoutMonthWeeks rows are contiguous and seven days wide`() {
+        val weeks = layoutMonthWeeks(jun, DayOfWeek.MONDAY, emptyList(), zone)
+        assertThat(weeks.first().days.first()).isEqualTo(jun1)
+        weeks.forEach { assertThat(it.days).hasSize(7) }
+        val allDays = weeks.flatMap { it.days }
+        allDays.zipWithNext { a, b -> assertThat(b).isEqualTo(a.plus(1, DateTimeUnit.DAY)) }
+    }
+
+    @Test
+    fun `a multi-day event becomes one span across its columns`() {
+        val ev = allDay(LocalDate(2026, 6, 10), LocalDate(2026, 6, 12))
+        val week = layoutCalendarWeek(weekOf8th, listOf(ev), zone)
+
+        assertThat(week.spans).hasSize(1)
+        val span = week.spans.single()
+        assertThat(span.startCol).isEqualTo(2) // Wednesday the 10th
+        assertThat(span.endCol).isEqualTo(4) // Friday the 12th
+        assertThat(span.lane).isEqualTo(0)
+        assertThat(span.continuesLeft).isFalse()
+        assertThat(span.continuesRight).isFalse()
+    }
+
+    @Test
+    fun `a span running past the row end is flagged as continuing`() {
+        // Saturday the 13th through Tuesday the 16th straddles the row boundary.
+        val ev = allDay(LocalDate(2026, 6, 13), LocalDate(2026, 6, 16))
+        val week = layoutCalendarWeek(weekOf8th, listOf(ev), zone)
+
+        val span = week.spans.single()
+        assertThat(span.startCol).isEqualTo(5)
+        assertThat(span.endCol).isEqualTo(6)
+        assertThat(span.continuesLeft).isFalse()
+        assertThat(span.continuesRight).isTrue()
+
+        // The following row picks it up with the flags mirrored.
+        val nextRow = layoutCalendarWeek(
+            weekOf8th.map { it.plus(7, DateTimeUnit.DAY) },
+            listOf(ev),
+            zone,
+        )
+        val tail = nextRow.spans.single()
+        assertThat(tail.startCol).isEqualTo(0)
+        assertThat(tail.endCol).isEqualTo(1)
+        assertThat(tail.continuesLeft).isTrue()
+        assertThat(tail.continuesRight).isFalse()
+    }
+
+    @Test
+    fun `overlapping spans are stacked on separate lanes`() {
+        val a = allDay(LocalDate(2026, 6, 9), LocalDate(2026, 6, 11), id = 1L)
+        val b = allDay(LocalDate(2026, 6, 10), LocalDate(2026, 6, 12), id = 2L)
+        val week = layoutCalendarWeek(weekOf8th, listOf(a, b), zone)
+
+        assertThat(week.spans.map { it.lane }).containsExactly(0, 1)
+    }
+
+    @Test
+    fun `single-day timed events stay pills and sort by start`() {
+        val late = timed(LocalDate(2026, 6, 10), 14, 15, id = 1L)
+        val early = timed(LocalDate(2026, 6, 10), 9, 10, id = 2L)
+        val week = layoutCalendarWeek(weekOf8th, listOf(late, early), zone)
+
+        assertThat(week.spans).isEmpty()
+        assertThat(week.timedByDay[LocalDate(2026, 6, 10)]?.map { it.instanceId })
+            .containsExactly(2L, 1L)
+            .inOrder()
+        assertThat(week.timedByDay[LocalDate(2026, 6, 11)]).isEmpty()
+    }
+
+    @Test
+    fun `countByDay totals bars and pills on each date`() {
+        val span = allDay(LocalDate(2026, 6, 10), LocalDate(2026, 6, 12), id = 1L)
+        val pill = timed(LocalDate(2026, 6, 10), 9, 10, id = 2L)
+        val week = layoutCalendarWeek(weekOf8th, listOf(span, pill), zone)
+
+        assertThat(week.countByDay[LocalDate(2026, 6, 10)]).isEqualTo(2)
+        assertThat(week.countByDay[LocalDate(2026, 6, 11)]).isEqualTo(1)
+        assertThat(week.countByDay[LocalDate(2026, 6, 13)]).isEqualTo(0)
+    }
+
+    @Test
+    fun `layoutMonthWeeks agrees with laying each row out on its own`() {
+        val events = listOf(
+            allDay(LocalDate(2026, 6, 10), LocalDate(2026, 6, 12), id = 1L),
+            timed(LocalDate(2026, 6, 18), 9, 10, id = 2L),
+        )
+        val weeks = layoutMonthWeeks(jun, DayOfWeek.MONDAY, events, zone)
+
+        weeks.forEach { row ->
+            assertThat(row).isEqualTo(layoutCalendarWeek(row.days, events, zone))
+        }
+    }
+
+    @Test
+    fun `instancesByDay puts all-day events first then orders by start`() {
+        val allDayEv = allDay(LocalDate(2026, 6, 10), id = 1L)
+        val late = timed(LocalDate(2026, 6, 10), 14, 15, id = 2L)
+        val early = timed(LocalDate(2026, 6, 10), 9, 10, id = 3L)
+
+        val byDay = instancesByDay(weekOf8th, listOf(late, early, allDayEv), zone)
+
+        assertThat(byDay.getValue(LocalDate(2026, 6, 10)).map { it.instanceId })
+            .containsExactly(1L, 3L, 2L)
+            .inOrder()
+    }
+
+    @Test
+    fun `instancesByDay repeats a multi-day event on every date it covers`() {
+        val ev = allDay(LocalDate(2026, 6, 10), LocalDate(2026, 6, 12))
+        val byDay = instancesByDay(weekOf8th, listOf(ev), zone)
+
+        assertThat(byDay.getValue(LocalDate(2026, 6, 9))).isEmpty()
+        assertThat(byDay.getValue(LocalDate(2026, 6, 10))).hasSize(1)
+        assertThat(byDay.getValue(LocalDate(2026, 6, 11))).hasSize(1)
+        assertThat(byDay.getValue(LocalDate(2026, 6, 12))).hasSize(1)
+        assertThat(byDay.getValue(LocalDate(2026, 6, 13))).isEmpty()
+    }
+
+    @Test
+    fun `instancesByDay covers every date in the grid, empty ones included`() {
+        val byDay = instancesByDay(weekOf8th, emptyList(), zone)
+        assertThat(byDay.keys).containsExactlyElementsIn(weekOf8th)
+        assertThat(byDay.values.flatten()).isEmpty()
+    }
+}